Krypton-Fluoride (KrF) excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. The KrF laser produces a laser beam having a narrow-band wavelength of about 248 nm and can be used to produce integrated circuits with dimensions as small as about 180 nm. The Argon Fluoride (ArF) excimer laser is very similar to the KrF laser. The primary difference is the laser gas mixture and a shorter wavelength of the output beam. Basically, Argon replaces Krypton and the resulting wavelength of the output beam is 193 nm. This permits the integrated circuit dimensions to be further reduced to about 120 nm. F2 lasers have long been recognized as the successor to the KrF and ArF lasers in the integrated circuit lithography industry since the F2 beam at 157 nm permits a substantial improvement in pattern resolution. These F2 lasers can be very similar to the KrF and ArF excimer lasers and with a few modifications it is possible to convert a prior art KrF or ArF laser to operate as an F2 laser. A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIG. 1 and FIG. 2. A cross section of the laser chamber of this prior art laser is shown in FIG. 3. A pulse power system 2 powered by high voltage power supply 3 provides electrical pulses to electrodes 6 located in a discharge chamber 8. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 inches per second. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module 18. Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module,
Pulse Power System with: high voltage power supply module,
commutator module and high voltage compression head module,
Output Coupler Module,
Line Narrowing Module,
Wavemeter Module;
Computer Control Module,
Gas Control Module,
Cooling Water Module
Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supported in this prior art embodiment by anode support bar 44 which is shown in cross section in FIG. 3. Flow is clockwise in this view. One corner and one edge of anode support bar 44 serves as a guide vane to force air from blower 10 to flow between electrodes 6A and 6B. Other guide vanes in this prior art laser are shown at 46, 48 and 50. Perforated current return plate 52 helps ground anode 6B to the metal structure of chamber 8. The plate is perforated with large holes (not shown in FIG. 3) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow. A peaking capacitor comprised of an array of individual capacitors 19 is charged prior to each pulse by pulse power system 2. During the voltage buildup on the peaking capacitor, two preionizers 56 weakly ionize the lasing gas between electrodes 6A and 6B and as the charge on capacitors reach about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 inch per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of subsequent pulses are close to the desired energy. In prior art systems, this feedback signal is an analog signal and it is subject to noise produced by the laser environment. This noise can result in erroneous power supply voltages being provided and can in turn result in increased variation in the output laser pulse energy.
These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance. One problem experienced with these prior-art lasers has been excessive wear and occasional failure of blower bearings.
A need exists in the integrated circuit industry for a modular, reliable, production line quality F2 laser in order to permit integrated circuit resolution not available with KrF and ArF lasers.
The present invention provides a reliable, modular, production quality F2 excimer laser capable of producing, at repetition rates in the range of 1,000 to 2,000 Hz or greater, laser pulses with pulse energies greater than 10 mJ with a full width half, maximum bandwidth of about 1 or less. Preferred embodiments of the present invention can be operated in the range of 1000 to 4000 Hz with pulse energies in the range of 10 to 5 mJ with power outputs in the range of 10 to 40 watts. Using this laser as an illumination source, stepper or scanner equipment can produce integrated circuit resolution of 0.1 xcexcm or less. Replaceable modules include a laser chamber and a modular pulse power system.
Important improvements over prior art excimer lasers have been provided in the pulse power unit to produce faster charging. These improvements include an increased capacity high voltage power supply, an improved communication module that generates a high voltage pulse from capacitors charged by the high voltage power supply and amplifies the pulse voltage about 23 times with a very fast voltage transformer having a secondary winding consisting of a single four-segment stainless steel rod. A novel design for the compression head saturable inductor (referred to herein as a xe2x80x9cpots and pansxe2x80x9d design) greatly reduces the quantity of transformer oil required and virtually eliminates the possibility of oil leakage which in the past has posed a hazard.
Improvements in the laser chamber permitting the higher pulse rates and improved performance include the use of a single preionizer tube.
In a preferred embodiment the laser was tuned to the F2 157.6 nm line using a set of two external prisms. In a second preferred embodiment the laser is operated broad band and the 157.6 nm line is selected external to the resonance cavity. In a third preferred embodiment a line width of 0.2 pm is provided using injection seeding.
Other embodiments of the present invention include ceramic bearings. Optionally magnetic bearings may be utilized. Reaction forces on the bearings may be reduced by providing an aerodynamic contour on the anode support bar. Other improvements include use of acoustic baffles for laser chambers producing disruptive acoustic shock waves.
Preferable the intracavity beam line and the output beam line fully sealed and nitrogen purged.